This invention relates to the field of mixing a gas in a liquid. The basic problem is to get as large a fraction of the injected gas into solution as possible for as long a retention time as possible. This objective is accomplished in part by entraining the gas as microfine bubbles within the liquid so that the area of the liquid-gas interface is maximized.
A recent process and device for so entraining a gas were disclosed by Damann, U.S. Pat. No. 4,735,750. In Damann's combination, gas is brought into contact with a liquid after it is sprayed through a plurality of small nozzles having throat diameters of only 2 mm. The use of such small nozzles, however, gives rise to the problem of nozzle clogging in some applications. In the oxygenation of pond water, for example, Damann's device has been found, in repeated tests, to clog readily with detritus, algae, grass and fish waste.
Not only are the sizes of the gas bubbles in a gas/liquid suspension a function of the degree of mixing (which is related to the nozzle size and configuration) but also of the static pressure of the gas/liquid mixture. When the pressure is high, the gas bubbles tend to be smaller and to have longer retention times in the liquid. As the pressure of the mixture decreases, the bubbles tend to expand, coalesce, float to the surface and escape the liquid.
For use in shallow fish ponds, the larger the bubbles released from an aeration device the higher the probability that the gas might escape the pond before the gas ever has a chance to be utilized there. The cited prior art teaches, however, that to keep the proper fluid level in the aeration device, a control valve positioned a considerable distance upstream of the actual discharge outlet should be used. Such a position for the control valve leads to a lowering of the pressure of the gas/liquid mixture aft of the valve, thereby allowing the gas bubbles entrained in the mixture to enlarge.